Compounding the issue is Facebook’s history of hesitating to address misinformation until a particular subject has snowballed into an urgent problem. In the middle of a global measles outbreak last year spurred by low vaccination rates, Facebook rolled out a series of policies to curb vaccine misinformation. But the changes did little to prevent the problem from resurfacing again amid the Covid-19 pandemic.

Concerned that Facebook’s failure to crack down on vaccine falsehoods will backfire at the worst possible time, advocates and scientists have called on Facebook co-founder Mark Zuckerberg to take action.

“Covid-19 misinformation is the equivalent of an ideological dirty bomb: It has the capacity to hurt tens of thousands of people when it detonates in the moment that vaccines are available,” said Imran Ahmed, founder and chief executive officer of the U.K.-based nonprofit Center for Countering Digital Hate, which recently assessed the growing influence of anti-vaccination content on social media platforms including Facebook since the outset of the pandemic.